The invention relates to a frequency synthesizer intended to receive an input signal having an average frequency referred to as symbol frequency, a clock signal having a fixed frequency, and to supply an output signal having a frequency referred to as output frequency, said synthesizer comprising:
a phase/frequency detector intended to compare the symbol frequency with a predetermined fraction of the output frequency and to supply a control signal for adjusting the value of said output frequency, PA1 a low-pass filter having an output and an input intended to receive the control signal, PA1 an oscillator having a control input which is connected to the output of the low-pass filter and an output which is intended to supply the output signal of the synthesizer whose output frequency depends on the signal at the control input. PA1 a time delay module intended to supply a signal referred to as end-of-cycle signal assuming an active state at the end of a period referred to as counting cycle which is equal to the multiplication of the duration of a period of the output signal of the synthesizer by a first predetermined digital value, PA1 a counting device having a clock input which is intended to receive the clock signal, an output having a value which is incremented, when the counting device is activated, at each new period of said clock signal, the counting device being only activated when the input signal of the synthesizer has an active logic state, the value of the output of the counting device being re-initialized when the end-of-cycle signal is in the active state, PA1 comparison means allowing a comparison of the value obtained at the output of the counting device at the of the counting cycle with a second predetermined digital value, the result of this comparison being used for generating the control signal. PA1 a module intended to generate a clock signal within the device, PA1 a tuner allowing selection of a data signal from the network, PA1 a digital demodulator rhythmed by the clock signal and intended to receive the data signal from the network and to generate, on the basis of said signal, a signal having an average frequency referred to as symbol frequency and a demodulated data signal, PA1 a first and a second register intended to comprise a first and a second predetermined value, respectively, PA1 a frequency synthesizer intended to generate a signal having a frequency referred to as output frequency, the ratio between the output frequency and the symbol frequency being substantially equal to the ratio between the first and second predetermined values, PA1 a digital modulator rhythmed by the output signal of the frequency synthesizer and intended to transmit another data signal to the network, PA1 characterized in that the frequency synthesizer is a synthesizer as described above.
A frequency synthesizer of this type is known from European patent application EP 0 727 877 A2. This synthesizer comprises first and second programmable dividers which supply signals at the output whose frequencies have a first and a second software-programmable division rate with respect to the output frequency and with respect to the symbol frequency, respectively. The frequencies of the output signals of the programmable dividers are compared by means of a phase/frequency detector which supplies a control signal intended to adjust the output frequency in order that the ratio between this frequency and the symbol frequency is equal to the ratio between the first and second division rates. Here, the input signal of this synthesizer is a reference signal having a frequency which is known and invariable and constituting the symbol frequency. In certain applications, particularly in the reception of signals from a communication network, the input signal, elaborated on the basis of said signals, may have a phase noise which becomes manifest by variations of the cyclic ratio and of the frequency around an average frequency which corresponds to the symbol frequency. The active edges of such a signal can thus no longer serve as reference pulses, as is the case in the known frequency synthesizer described above.